The native Camptothecin (“CPT”) has a pentacyclic structure of a fused ring system consisting of quinoline rings (Ring A and Ring B), a pyrrolidine ring (Ring C), an alpha-pyridone ring (Ring D), and a six-membered lactone ring (Ring E). CPT has only one asymmetric center at 20-position and displays dextro-rotation due to the S-configuration of a tertiary hydroxyl group. CPT is a cytotoxic alkaloid which was first isolated and characterized by Wall and his coworkers (J. Am. Chem. Soc. 88, 3888, 1966) from leaves and barks of Camptotheca accuminata (NYSSACEAE), a plant native to China. The primary cellular target for CPT is topoisomerase I (topo I), an enzyme involved in the relaxation of supercoiled chromosomal DNA during DNA replication by transient single-strand cleavage of duplex DNA, unwinding and religation. CPT binds at the interface of covalent binary topo I-DNA complex to form stable ternary complex, which prevents the religation of DNA after the unwinding, and consequently leads to replication-mediated double-strand breaks and DNA damage. Because CPT inhibition can lead to cell death during S-phase of the cell cycle, CPT has become the focus of extensive studies in anticancer drug development (Nature Review/Cancer, October 2006 Vol. 6, pp 789-802; Bioorg. Med. Chem., 2004, 12, pp 1585-1604).
The native CPT is not soluble in water or in other aqueous vehicles that are suitable for parental administration. At pH 7 or above, the E-ring lactone structure of CPT can be hydrolyzed to form the ring-opened carboxylate derivative, which is water-soluble but lacks of the biological activity required and exhibits high clinical toxicity. At the physiological condition, the E-ring lactone hydrolysis reaction may be exacerbated due to the preferential binding (150-fold higher than CPT) of the carboxylate derivative to human serum album (J. Med. Chem. 1993, 36, 2580; Anal. Biochem. 1993, 212, 285; Biochemistry, 1994, 33, 10325; Biochemistry, 1994, 33, 10325; Pharm. Sci. 1995, 84. 518). The water-insolubility of CPT and the clinical toxicity of its carboxylate derivative are two limiting factors preventing CPT from being used as an antitumor chemotherapeutic agent in clinical applications (Nature Review/Cancer, October 2006 Vol. 6, pp 789-802). It thus would be desirable to find CPT derivatives with better in vivo lactone stability and water-solubility than native CPT (Bioorg. Med. Chem., 2004, 12, pp 1585-1604; Chem. Rev., 2009, 109 (1), pp 213-235).
In literature, the attempts to develop the bioactive CPT analogs with better water solubility have been focused on introducing hydrophilic groups to the A, B, or/and C ring(s) of CPT (Bioorg. Med. Chem., 2004, 12, pp 1585-1604; Chem. Rev., 2009, 109 (1), pp 213-235). Compared to the native CPT, attaching chemical modifying groups to the fused ring system would, to some extent, adversely affect CPT's binding to the surface of covalent binary topo I-DNA complex to form stable tertiary complex. As a result, the bioactivity of these CPT analogs (e.g. Topotecan, which is used as a standard anticancer drug to inhibit cancer cell growth) is generally less than that of CPT (Nature Review/Cancer, October 2006 Vol. 6, pp 789-802; Bioorg. Med. Chem., 2004, 12, pp 1585-1604). On the other hand, chemical modification at the A, B, C rings of CPT cannot mitigate the hydrolysis of the E-ring lactone. It is generally believed that the E-ring lactone hydrolysis is facilitated by the hydrogen bonding interaction between the 20(S)-hydroxyl group and the neighboring carbonyl group (Bioorg. Med. Chem., 2004, 12, pp 1585-1604; Chem. Rev., 2009, 109 (1), pp 213-235). Previous literature has shown that, in order to increase CPT lactone ring stability, one approach is to disrupt the hydrogen bond interaction between the 20(S)-hydroxyl and the neighboring carbonyl, e.g. by reaction of the 20(S)-hydroxyl with alkyl or acyl to form ether or ester, thereby preventing acceleration of the E-ring lactone hydrolysis. However, the 20(S)-hydroxyl group is essential for the pharmacological activity of CPT. The CPT analogs without the 20(S)-hydroxyl group generally are proven to lack of antitumor efficacy (Organic Lett., 2004, 6(3), pp 321-324; Bioorg. Med. Chem., 2004, 12, pp 1585-1604; Chem. Rev., 2009, 109 (1), pp 213-235).
From the above discussion, the strategy to attach a water-soluble prodrug group (e.g. ionized functional group) to the 20(S)-hydroxyl site would be a practical approach to increase the water-solubility of the resulting prodrug molecule (feasibility of drug administration) while improving the E-ring lactone stability of the CPT prodrug in blood during circulation (clinical safety of the drug). By doing so, this prodrug approach would convert the water-insoluble CPT molecule to the water-soluble CPT prodrug; because such a water-soluble CPT prodrug could quickly diffuse to the whole human body after entering the blood stream, the CPT prodrug would exist in very low concentration during the metabolism, thereby preventing precipitation of CPT in the blood vessels. In addition, by introducing a screening prodrug group at the 20(S)-hydroxyl site, the hydrogen bond interaction between the 20(S)-hydroxyl and the neighboring carbonyl, which would facilitate the E-ring lactone hydrolysis, could be prevented. As a result, the E-ring lactone stability of the CPT prodrug in blood stream during circulation could be enhanced, and the clinical drug safety concerns, e.g. hematotoxicity related to carboxylate derivative generated by CPT hydrolysis, could be mitigated. Obviously, the prodrug approach of protecting the 20(S) hydroxyl site with a water-soluble prodrug group is a medicinal chemistry method which can bring in lactone stability, water-solubility and bioactivity to facilitate CPT anticancer drug development.
The attempts to prepare the CPT prodrugs or CPT-based compounds by chemical modification of the 20(S)-hydroxyl site have been reported in literature. Among them, most efforts were to introduce various protecting functional groups (including lipophilic and charged functional groups) through esterification of the 20(S)-hydroxyl group (Chem. Rev., 2009, 109 (1), pp 213-235). Conversion of the ester prodrug to the native CPT is mediated by a group of enzymes known as esterases, which exist widely in the blood of animals (including humans). The shortcoming of the ester prodrugs is the relatively poor stability of the ester linkage in human body at physiological condition, which is easy to break by esterases. The clinical benefit of the CPT ester prodrug approach was not promising (Chem. Rev., 2009, 109 (1), pp 213-235). In another attempt, the CPT 20(S)—O-phosphonate esters have been prepared (Organic Lett., 2004, 6(3), pp 321-324). The disclosed 20(S)—O-phosphonates could improve water-solubility and in vivo lactone stability of CPT, but as tested in experiments, the CPT derivatives of 20(S)—O-phosphonates lack of antitumor activities (Organic Lett., 2004, 6(3), pp 321-324). The 20(S)—O-phosphonate esters cannot be converted to CPT at the physiological conditions (Organic Lett., 2004, 6(3), pp 321-324).
It thus would be still desirable to develop CPT derivatives which have acceptable water-soluble and E-ring lactone stability, as well as good anticancer efficacy.